Venus

Space Sciences
COPYRIGHT 2002 The Gale Group Inc.

Venus

Venus was one of the last planets to be explored, despite its position as the closest planet to Earth. This is largely because it is perpetually shrouded in a uniformly bland covering of clouds. The cloud cover made looking at Venus through a telescope about as exciting as staring at a billiard ball. While Mars and the Moon were objects of much attention by early telescopic observation, the surface of Venus remained a mystery. It was even easier to say something about the outer planets, such as Jupiter and Saturn, than it was to make meaningful observations of Venus.

The absence of information about Venus was particularly ironic because Venus is the most like Earth in size and position within the solar system, thus suggesting that it could be more like Earth than any of the other planets. Venus's diameter is only 651 kilometers (404 miles) smaller than Earth's diameter of 12,755 kilometers (7,908 miles). Venus's density is 0.9 times that on Earth, and its surface gravity is 0.8 that of Earth. Venus orbits the Sun in just under one Earth year (224.7 days). When compared to Earth, all of the planets except Venus are much larger or smaller, higher or lower in density, located at much greater or lesser distances from the Sun, or enveloped in atmospheres much thinner or colder. Thus Venus was a cornerstone in scientists' survey of the solar system and offered the chance to see how an Earth-sized planet might have evolved similarly or differently. Planetary geologists now know that it is very different. This fact has revealed that the details of how a planet geologically evolves are probably as important in planetary evolution as differences in fundamental characteristics. Venus and Earth are truly twins separated at birth.

Atmospheric Characteristics

Because of the cloud cover, one of the first things that could be determined about Venus in the early days of planetary astronomy was the characteristics of the visible atmosphere. This was done first through telescopic measurements and early spacecraft flybys . In the nineteenth century, rare transits of Venus across the surface of the Sun were used to prove that Venus was enveloped in an atmosphere. This led to all sorts of early speculation that the clouds were, like clouds on Earth, water vapor clouds, and that the surface was a teeming primordial swamp filled with plants and animals similar to the Paleozoic coal swamps of Earth. This speculation withered under results of early spectroscopic observations, which were able to determine that the atmosphere was largely carbon dioxide, not oxygen and nitrogen as on Earth, and later on, that the clouds appeared to be sulfuric acid fog, not water vapor.

By the 1960s little was still known about Venus, but modern instruments were beginning to reveal more. Early surface temperature estimates were made by observing infrared wavelengths to better determine the temperature. Such observations, using radio telescopes and the first U.S. inter-planetary flyby spacecraft, Mariner 2, in 1962, implied that the surface temperatures were high. Over the next decade, several U.S. atmospheric entry probes (Pioneer-Venus 1 and 2) and Soviet landers (Venera 9, 10, 13, and 14) directly measured the temperature and pressure within the atmosphere. These measurements revealed a surface temperature of 450°C
(842°F), or about as hot as the surface of a catalytic converter on an automobile. The surface pressure on Venus was found to be ninety-two times that of Earth (92 bars or 9.2 million pascals). This is equivalent to pressures at about 1 kilometer (0.6 mile) of depth in the sea, or about fifty times greater than a pressure cooker.

The atmosphere is so dense that pressures and temperatures similar to the surface of Earth occur at about 60 to 70 kilometers (37 to 43 miles) of altitude. Most of the atmosphere on Earth lies below 10 kilometers (6.2 miles), and it pretty much peters out before 30 kilometers (18.6 miles). On Venus, a daring future adventurer in a balloon with an oxygen mask (and protection from sulfuric acid clouds) could float in the upper atmosphere at an altitude of 60 to 70 kilometers (37 to 43 miles) in relative comfort. The global trip would be rapid since the atmosphere super-rotates, meaning that it flows from west to east faster than the underlying surface rotates. A balloon traveler in the upper atmosphere of Venus could circumnavigate the planet in only four days, especially near the equator where the speeds are greatest. This is unlike Earth, where the surface spins beneath a relatively sluggishly moving atmosphere that takes several weeks for a complete circuit. The balloon traveler's view would be boring, however, because the surface of Venus would be obscured by the main cloud layer, which occurs at 45 to 70 kilometers (28 to 43 miles) of altitude.

These conditions are the result of early development of a thick atmosphere consisting mostly of carbon dioxide (about 97 percent) through a
so-called runaway greenhouse effect. First discovered from the study of Venus, the greenhouse effect is now discussed for Earth, where it is recognized that industrial additions of carbon dioxide to the atmosphere pose potential environmental problems of similar global magnitude.

Surface Features and Geologic Findings

Although the surface of Venus has been seen locally around a few Russian landers with optical cameras, a true picture of the global surface was obtained only with the advent of radar that could penetrate the dense obscuring clouds and create radar images. Early results were obtained through Earth-based radio telescopes at Goldstone (in California) and Arecibo (in Puerto Rico), which emitted tight beams of radar and built up images showing differences in the radar reflecting properties of the surfaces. These images were low in resolution, but they enabled large areas of unusually radar-reflective terrain to be detected. These also allowed the first estimates of the rotation to be made, and they showed that Venus rotates backwards, or west to east, and slowly. It takes about 243 days to do so. Oddly, this is a little longer than its year (224.7 days). Stranger still, at closest approach to Earth (a distance of just under 40 million kilometers [24.8 million miles]), or opposition, Venus presents the same hemisphere to Earth. The origin of this unusual set of rotation conditions is not known.

The first truly global maps of Venus was made by the Pioneer-Venus orbiter using radar altimetry . This showed the surface elevations over the globe resolved at scales of about 100 kilometers (62 miles) and revealed a relatively flat surface, with the absolute range of elevations much less than that found on Earth. More than 80 percent of the surface area lies within a kilometer of the mean planetary radius (6,051.8 kilometers [3,752.1 miles]). A few highland regions rise from one to several kilometers above the mean planetary radius, but these cover only about 15 percent of the surface of Venus. Whereas Earth has two common elevations, seafloors and continents, Venus has one most common elevation, broad plains.

Radar images of the surface, somewhat similar to photographs, were made later for large areas of the globe by the Russian Venera 15 and 16 orbiters, for the northern quarter of Venus, and, several years later, by the U.S. Magellan orbiter, for about 98 percent of the surface. These efforts obtained images of the surface at scales of 2 kilometers (1.2 miles) and 0.3 kilometers (0.2 miles), respectively, thus generating the first true images of what the surface of Venus really looks like and permitting the first geologic analysis.

The radar images show that the surface of Venus is a complex of plains, mountains, faults , ridges, rift valleys , volcanoes, and a few impact craters , a surface more complex and geologically modified than any of the other planets seen previously. The highland regions seen first in low-resolution Earth-based radar images and Pioneer-Venus radar altimetry are among the most complex surfaces and consist of a terrain that is complexly faulted in orthogonal patterns. These regions are known as tessera after the Greek word for mosaics of tiles. The sequence of geologic surfaces suggests that tesserae (plural of "tessera") also represent the oldest preserved surfaces on Venus. One of the highlands is surrounded by ridgelike mountain belts that rise from 6 to 11 kilometers (3.7 to 6.8 miles) above the mean planetary radius
and appear to have formed from compression and buckling of the surface, similar to mountain belts on Earth. Low ridges of possibly similar origin, in a range of sizes, occur singly or in belts throughout the plains areas.

Faults, fractures , and immense rift valleys are present in abundance. One rift valley, Diana Chasma, is similar in size to the great East African rift valley and Rio Grande rift valley of Earth. Like those on Earth, it probably formed from the stretching and pulling apart of the surface. On Earth, erosion and sedimentation quickly obscure all but the latest structures associated with such rifts. But on Venus the absence of erosion means that all of the structural details are perfectly preserved as a complex mass of faults.

Large volcanoes up to several hundreds of kilometers across but only a few kilometers high are common, as are long lava flow fields, extensive lowlying regions of lava plains, and lava channels. One lava channel is longer than the largest rivers on Earth. Low-relief domical volcanoes, many less than several kilometers in diameter, are globally abundant, numbering in the hundreds of thousands. Some volcanoes appear similar to those formed from eruption of thick, viscous lavas on Earth. Additional volcanic features include calderas similar to those on Earth, although generally much larger; complex topographic annular spider-and-web-shaped features known as arachnoids; and circular structural patterns up to several hundred kilometers across with associated volcanism known as coronae. Many of these are generally thought to represent local formation of large and deep magma reservoirs. Radial patterns of fractures associated with volcanoes are common and may represent the surface deformation associated with radial-dike-like magma intrusions.

Impact craters are about as numerous on Venus as they are on continental areas of Earth, and are thus not as common as they are on most other planets. Only about 900 have been identified on Venus. Meteors smaller than a certain size disintegrate on entering the atmosphere. As a result, impact craters smaller than 2 kilometers (1.2 miles) are infrequent. Morphologically, craters on Venus resemble those on other planets with several exceptions related to the interaction of the crater ejecta with the dense atmosphere. These include extensive parabola-shaped halos much like fallout from plumes associated with volcanic eruptions on Earth. These open to the west and possibly record the interaction of the upward expanding cloud of crater ejecta with the strong global easterly winds. Many craters are characterized by large lava-flow-like features that may represent molten ejecta flowing outward from the crater after the impact.

Impact craters also appear nearly uniformly distributed, unlike most planets where large areas of different crater abundance indicate variations in age of large areas of their surfaces. Based on estimates of their rates of formation on surfaces in the inner solar system, impact crater statistics indicate an average surface age on Venus of about 500 million years. Either most of the surface was formed over 500 million years ago in a catastrophic resurfacing event and volcanism has been much reduced since that time, or continual, widespread, and evenly spaced volcanism and tectonism remove craters with a rate that yields an average lifetime of the surface of 500 million years. The rate of volcanism on Venus is estimated to be less than 1 cubic kilometer per year, somewhat less than the 20 cubic kilometers associated largely with seafloor spreading on Earth. The surface of Venus
appears to be dominated by volcanic hot spots rather than spreading and subduction associated with plate tectonics.

Another spacecraft observation method allowed something to be determined about the interior of Venus. By carefully tracking spacecraft orbits, variations in gravitational acceleration associated with differences in mass on and beneath the surface can be detected. On Venus, this technique reveals that the strength of gravity is mostly proportional to the surface topography, in contrast to Earth, where mass associated with topography is generally compensated underneath by lower density roots. This means that many large topographic features on Venus are supported either by strong lithosphere without a low-density root, or by topography originating from the dynamical uplift of the surface through convective processes in the deep interior. If the first type is assumed, it may indicate that the lithosphere is strong and that a low-strength layer at the base of the lithosphere (called the asthenosphere on Earth) is not present. The second type may be attributed to upwelling associated with volcanic hot spots.

Several Venera landers of the Russian space program returned both optical images of the surface and chemical information about the rocks at several sites. Early landers had searchlights in case the cloud cover made it too dark to see anything. Despite the dense cloud cover, enough light gets through that the surface is illuminated to the equivalent of a cloudy day on Earth. But the sky as seen from the surface is probably a bland fluorescent yellow-white, rather than mottled gray. The relatively rocky surroundings appeared to be volcanic lava flow surfaces or associated rubble. The measured chemical compositions are indistinguishable for the most part from tholeiitic and alkali basalts typical of ocean basins and hot spots on Earth.

The low number of impact craters scattered over the surface implies that only the last 20 percent of the history of Venus appears to be preserved, and little is known about the earlier surface geologic history. The geological complexity and young surface ages of both Venus and Earth relative to smaller terrestrial planets can be attributed to their larger sizes and correspondingly warmer and more mobile interiors, extensive surface deformation (tectonism), and mantle melts (volcanism) over a greater period of geological time.

Venus

Venus The goddess Venus represents the ideal of seductive female beauty. In her Greek form, Aphrodite, or as the Roman Venus, she is associated with the seduction of mortals by gods, and with sexual relationships between mortal men and women. In Greek myth Aphrodite emerged from the sea after the castration of Kronos, the Titan. The etymology of the Latin name ‘Venus’ is unclear, although it may relate to words for both ‘charm’ and ‘poison’; but, before the elision of the Greek and Roman deities late in the third century bc, the Italian goddess Venus seems to have been associated with the fertility of gardens rather than with human sexuality. Her special importance to the Romans was increased by her role as mother of the hero Aeneas, one of the legendary figures associated with the foundation of the city of Rome. Venus mediates between Jupiter, head of the Roman pantheon, and the Roman people.

In the Roman republic, several generals claimed to be under her personal protection, including Sulla, who marched on Rome and took the city in 88 bc. Most famous of these generals was Julius Caesar, whose family claimed direct descent from Venus; he promoted her cult, especially as Venus Victrix, ‘she who conquers’, and he built a new temple to the goddess. The first imperial dynasty of the Roman Empire, the Julio-Claudians, emphasized both their links to Julius Caesar and their right to rule through their continued emphasis on Venus.

Aphrodite/Venus has been a popular subject in art since the classical period, providing a rationale for showing the naked female body in a variety of poses in periods when it would be considered inappropriate to represent a real woman in this way. A particularly common theme in post-classical art is ‘The Toilet of Venus’, showing her with Eros, her son by the war-god Ares, holding up a mirror in which she can admire her own beauty. Other standard poses represent her as a modest young woman about to take a bath, rising from the sea, or wringing her hair out on the beach. In many of these poses she is represented as if she thinks she is unobserved; the observer is thus cast as a voyeur. Sometimes she shields her breasts and pudenda as if she has been startled to find that an onlooker is present, in a pose known as the Venus pudens that paradoxically only draws attention to the parts which are concealed.

In the Middle Ages, Venus was used to represent the sin of luxuria or sensuality; in battles for the soul, she was invariably lined up on the side of the vices. However, the Italian Neoplatonists saw love as a metaphysical experience transforming the soul and bringing awareness of the divine, so that images of Venus could be used to suggest divine rather than secular love and union.

The damaged marble image of Aphrodite found on Melos in 1820 (the Venus de Milo), perhaps the most famous statue in the history of the nude, dates to the second century bc; Man Ray's version, Venus Restored, represents her tied with rope. A description of the Venus de Milo by the classical scholar L. R. Farnell, written in 1896, shows the lengths to which Victorian writers went in playing down the sexuality of Venus; he claimed that she was ‘free from human weakness or passion’, ‘stamped with an earnestness lofty and self-contained, almost cold’.

Venus remains the ideal of female beauty, and as such she appears in some unlikely places. It is significant that Leopold von Sacher-Masoch chose to call his novel celebrating masochism Venus im Pelz (Venus in Furs, 1870), while in 1884, under the pseudonym ‘Rachilde’, Marguerite Eymery, who sometimes dressed as a man, published her Monsieur Venus, the story of a woman who uses the parts of her dead lover to create a male Venus.

Venus

UXL Encyclopedia of Science
COPYRIGHT 2002 The Gale Group, Inc.

Venus

Venus, the second planet from the Sun, is the closest planet to Earth. It is visible in the sky either three hours after sunset or three hours before sunrise, depending on the season. This pattern prompted early astronomers to refer to the planet as the "evening star" or the "morning star." Venus is named for the Roman goddess of love and beauty. Throughout history, the planet has been thought of as one of the most beautiful objects in the sky.

Venus and Earth have long been considered sister planets. The reason for this comparison is that they are similar in size, mass, and age. The diameter of Venus at its equator is about 7,500 miles (12,000 kilometers). The planet revolves around the Sun at an average distance of 67 million miles (107 million kilometers). It takes Venus about 225 Earth days to complete one revolution. The planet spins extremely slowly on its axis, taking about 243 Earth days to complete one rotation. Like Uranus and Pluto, Venus spins on its axis in the opposite direction to which it orbits the Sun.

Space probes to Venus

Beginning in 1961, both the United States and the former Soviet Union began sending space probes to explore Venus. The probes revealed that Venus is an extremely hot, dry planet, with no signs of life. Its atmosphere is made primarily of carbon dioxide with some nitrogen and trace amounts of water vapor, acids, and heavy metals. Its clouds are laced with sulfur dioxide.

Venus provides a perfect example of the greenhouse effect. Heat from the Sun penetrates the planet's atmosphere and reaches the surface. The heat is then prevented from escaping back into space by atmospheric carbon dioxide (similar to heat in a greenhouse). The result is that Venus has a surface temperature of 900°F (482°C), even hotter than that of Mercury, the closet planet to the Sun.

Under Venus's atmosphere, the U.S. and Soviet space probes found a rocky surface covered with volcanoes (some still active), volcanic features (such as lava plains), channels (like dry riverbeds), mountains, and medium- and large-sized craters.

Magellan. The U.S. probe Magellan mapped the entire Venusian surface from 1990 to 1994. The Magellan radar data showed that Venus is remarkably flat, and that some 80 percent of the planet's surface is covered by smooth volcanic plains, the result of many lava outflows. Magellan also revealed the existence of two large continent-like features on Venus. These features are known as Ishtar Terra (named after the Babylonian goddess of love) and Aphrodite Terra (named after the Greek goddess of love). Ishtar Terra, which measures some 620 miles (1,000 kilometers) by 930 miles (1,500 kilometers), lies in Venus's northern hemisphere. It has the form of a high plateau ringed with mountains. The largest mountain in the region, Maxwell Montes, rises to a height of 7 miles (11 kilometers). Aphrodite Terra is situated just to the south of the Venusian equator and is some 10,000 miles (16,000 kilometers) long by 1,200 miles (2,000 kilometers) wide. It is a region dominated by mountainous highlands and several large volcanoes.

Astronomers analyzing Magellan 's data have concluded that about 500 to 800 million years ago, lava surfaced and covered the entire planet,

giving it a fresh, new face. One indication of this event is the presence of volcanic craters and other formations on the surface that lack the same weathered appearance of that of older formations.

[See alsoSolar system ]

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Venus (in astronomy)

The Columbia Encyclopedia, 6th ed.

Copyright The Columbia University Press

Venus, in astronomy, 2d planet from the sun; it is often called the evening star or morning star and is brighter than any object in the sky except the sun and the moon. Because its orbit lies between the sun and the orbit of the earth, Venus passes through phases like those of the moon, varying from a large bright crescent when the planet is near inferior conjunction (nearest the earth) to a smaller silvery disk when it is at superior conjunction (farthest from the earth). Since its greatest elongation (the angle made between the sun, the earth, and Venus) is 47°, it can never be seen much longer than 3 hr after sunset or 3 hr before sunrise.

Venus revolves around the sun at a mean distance of c.67 million mi (107 million km) in a nearly circular orbit, and its period of revolution is about 225 days. It comes closer to the earth than any other planet, being c.26 million mi (42 million km) away at inferior conjunction. Venus is often referred to as the sister planet of the earth, because it is only slightly smaller in both size and mass. Several important differences, however, exist between the two planets.

Although Venus is covered with a thick blanket of clouds that hides its surface from view, much has been learned of the conditions on Venus from U.S. and Soviet space probes. These probes indicate a surface temperature of about 890°F (475°C) and an atmospheric pressure as great as 100 times that at the earth's surface. The thick atmosphere is composed mainly of carbon dioxide, with a slight amount of water vapor and a trace of nitrogen and other elements. The high surface temperature is assumed to result partly from the greenhouse effect; radiation passing through the atmosphere heats the surface, but the heat is blocked by the enveloping carbon dioxide from escaping back out through the atmosphere. The European Space Agency's Venus Express space probe began orbiting the planet in 2006; its instruments are designed primarily to study the Venusian atmosphere.

Studies also indicate that Venus rotates on its axis in a retrograde direction (opposite to the direction of revolution about the sun) with a period of about 243 days. Despite this slow rotation there is little observed temperature difference between the lighted and unlighted sides of the planet. The surface of Venus is thought to be stormy.

From 1990 to 1992 NASA's Magellan spacecraft mapped the Venusian surface using radar, revealing details of a continentlike feature, called Aphrodite Terra, that crosses the planet's equator and is marked by geologic faults. A second such feature, Ishtar Terra, straddles the north polar region. Magellan also observed many volcanic features, including immense lava plains and large shield volcanoes, and relatively few impact craters resulting from asteroids and comets. Compared to the number of craters on other bodies of the inner solar system, this suggests that the surface of Venus is only about 800 million years old. No strong magnetic field comparable to that of the earth has been detected.

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Venus

Venus Second planet from the Sun, it is almost as large as the Earth. Visible around dawn or dusk as the so-called morning star or evening star, it is the most conspicuous celestial object after the Sun and Moon. A telescope shows the planet's dazzling, yellowish-white cloud cover, with faint markings. In 1958 measurements at radio wavelengths indicated the very high surface temperature of Venus. Space probes revealed more about the surface. A gently undulating plain covers two-thirds of Venus. Highlands account for a further quarter, and depressions and chasms the remainder. Most of the surface features are volcanic in origin. The atmosphere consists of 96% carbon dioxide and 3.5% nitrogen, with traces of helium, argon, neon, and krypton. Venus has no satellites.